Sit-ups once ruled as the way to tighter abs and a slimmer waistline, while “planks” were merely flooring. Now planks — exercises in which you assume a position and hold it — are the gold standard for working your core, while classic sit-ups and crunches have fallen out of favor. Why the shift?
One reason is that sit-ups are hard on your back — by pushing your curved spine against the floor and by working your hip flexors, the muscles that run from the thighs to the lumbar vertebrae in the lower back. When hip flexors are too strong or too tight, they tug on the lower spine which can be a source of lower back discomfort.
Second, planks recruit a better balance of muscles on the front, sides, and back of the body during exercise than sit-ups, which target just a few muscles. Remember, your core goes far beyond your abdominal muscles.
Finally, activities of daily living, as well as sports and recreational activities, call on your muscles to work together, not in isolation. Sit-ups or crunches strengthen just a few muscle groups. Through dynamic patterns of movement, a good core workout helps strengthen the entire set of core muscles — the muscles you rely on for daily activities as well as sports and recreational activities.
Source: Harvard Medical School